Gas turbine engines include a compressor, a combustor, and a turbine coupled to the compressor. The combustor can include a plurality of combustor cans. Compressed air and fuel are delivered to the combustor cans to produce high-velocity and high-pressure combustion gases. These combustion gases are discharged to the turbine. The turbine extracts energy from the combustion gases for producing power that can be used in several ways such as, for example, to power the compressor, to power an electrical generator, or to power an aircraft. Stationary gas turbines can be divided into two categories; heavy duty industrial and aeroderivative.
Heavy duty gas turbines are designed specifically for ground based operation, where size and weight are not a constraint. Heavy duty gas turbines are designed for reliable operation and minimal maintenance at a competitively low installed cost. They accommodate a wide range of fuels including natural gas, light and heavy distillate oil, naphtha, crude, residual oil, among others.
An aeroderivative gas turbine is essentially an aircraft engine adapted for use in marine and industrial applications. The power generation capacity of these machines is typically in the 10 to 50 MW range. Aeroderivative gas turbines are relatively light construction, operate at high speeds, and generally use rolling element bearings. They have a narrower range of gaseous fuels that can be used.
There are significant variations between the aero-derivative and heavy industrial gas turbines. Among the variations are weight, combustor design, turbine design, and bearing design (including the lube-oil system). The primary distinction is in the bearing selection where the heavy duty gas turbines use hydrodynamic (journal) bearings and the aero-derivatives use anti-friction (ball or roller) bearings.
Hydrodynamic bearings and anti-friction bearings require lubrication to reduce friction and wear. In some cases, bearing assemblies include a supply pump that supplies lubricating oil under pressure to the bearing assemblies, and a scavenge pump that removes lubricating oil from the sump. In some cases lubrication of the bearings leads to leakage of the lubricant in the form of oil mist and vapor into the compressor. Any wetting of the blades or vanes by oil vapor will promote the accumulation of dust and dirt. A dirty blade or vane represents high friction-to-airflow that decreases engine efficiency, and results in a noticeable decrease in thrust or increase in fuel consumption.
To prevent oil leakage in some aeroderivative gas turbines the bearing may be disposed in a bearing housing within a pressurized assembly. The bearing housing and the pressurized assembly are isolated from the surrounding environment by labyrinth seals that extend around the rotor shaft. During operation, compressed air is supplied to the pressurized assembly to maintain a positive pressure around the bearing housing. This reduces oil leakage. In one example, disclosed in U.S. Pat. No. 6,470,666, an evacuation system that reduces the pressure within the bearing housing is provided. The evacuation system includes an air pump disposed downstream from an air/oil separator. During low power or idle operations, when there is insufficient air supply pressure from the bearing housing or cavity pressurization air supply, the air pump draws air from the bearing housing through the air/oil separator, such that an operating pressure within the bearing housing is reduced below the pressure within the pressurized assembly. For the system to operate successfully, the seals between the bearing housing and the pressurized assembly, and the seals between the pressurized assembly and the surrounding environment must be maintained. This configuration disclosed in U.S. Pat. No. 6,470,666 will function in a compromised manner in a heavy duty gas turbine application since the lube oil reservoir in a heavy duty gas turbine is vented and maintained at atmospheric pressure. Having the oil/vapor separator on the vacuum side of the evacuation pump will prevent the separated oil from flowing back to the lube oil reservoir due to the negative pressure differential. In a heavy duty gas turbine the negative pressure assembly must be capable of continuous duty, as compared to the system disclosed in U.S. Pat. No. 6,470,666, which operates during engine low-power and idle operations. Additionally, heavy-duty gas turbines are not equipped with a bearing housing pressurization air supply as that supply air flow constitutes a parasitic loss on the overall output and efficiency of the gas turbine.